The present invention relates to safety switching apparatuses for safely disconnecting an electrical load, and in particular to apparatuses for safely disconnecting an electrical machine. Even more specifically, the present invention relates to such a safety switching apparatus comprising at least one output switch for interrupting an external power supply path, the output switch having at least two switching positions, and comprising a control unit which controls and monitors the switching position of the output switch.
Prior art safety switching apparatuses are especially used in industry for at least partially disconnecting electrically driven machines, such as large brake presses, CNC-controlled machining centers or robots, in a reliable manner. A typical application is the disconnection of a machine when a guard door is opened, a light barrier is interrupted or an emergency off switch is actuated. The signals from these signal transmitters are fed to the prior art safety switching apparatuses which disconnect the corresponding machine or machine parts in response thereto. In this case, the actual disconnection process, i.e. the interruption of a power supply to the machine, must take place in an extremely reliable manner, since otherwise the health or even the life of the operator is put at risk. Prior art safety switching devices are therefore approved for operation by relevant regulatory authorities, such as professional associations (BG) in Germany, only if they meet the specific minimum requirements for their intrinsic failsafety. In general, a safety switching apparatus must allow to disconnect the machine in a reliable and failsafe manner even when faults or disruptions occur within the safety switching apparatus or the signal transmitters associated with it.
Therefore, only devices and systems that meet at least Category 3 of European Standard EN 954-1 or a corresponding safety standard are considered as safety switching apparatuses within the meaning of the present invention.
With prior art safety switching apparatuses, the switching position of the output switch, with which the power supply to the machine can be interrupted, must always be monitored in a reliable and failsafe manner. The reason for this is the fact that, otherwise, a fault or a disruption in the operation of the output switch may lead to the monitored machine being in a dangerous, unsafe state. For long and up to now, electromechanical switching elements, i.e. relays or contactors, have often been used for the output switches in prior art safety switching apparatuses. Such switching elements are known to be subject to the risk of the switching contacts welding together owing to sparking when the switch is opened. In this case, the output switch can no longer be opened, which may have devastating consequences.
Only in recent times have semiconductor components also been used as output switches in prior art safety switching apparatuses. Although mechanical contacts cannot weld together with these semiconductor components, there is basically the possibility of the switching path through the component breaking down and thus likewise of a short circuit forming which can no longer be opened.
WO 01/37302 A1 discloses a prior art safety switching apparatus which has two relays as output switches on the output side. The make contacts of the two relays are connected in series in a manner known per se in order to achieve redundancy and thus increased reliability on disconnection. Each of the two relays has two or more make contacts such that, for example, all three phases of a three-phase connection can be disconnected. In addition, each of the two relays has an auxiliary contact which is positively (forcibly) driven by the make contacts through a mechanical link. This mechanically positively driven operation ensures that the switching position of the make contacts corresponds to the switching position of the auxiliary contact. The switching position of the make contacts can thus be monitored by means of the auxiliary contact.
The described arrangement is widely used in practice and has proved very successful. One disadvantage, however, is that relays with positively driven contacts are considerably more expensive than basic relays. In addition, positively driven relays are larger and have lower holding forces than basic relays.